In a wind turbine generator, a rotor head having wind turbine blades receives wind force and rotates, the rotation is increased by a speed-up gear, and power is generated by a power generator which is driven by the increased rotation.
The rotor head is mounted on a tower for a wind turbine (hereinbelow, called “tower”), attached to an end of a nacelle which can yaw, and supported so as to be rotatable about the rotary axis in an substantially horizontal direction.
Generally, the tower for a wind turbine often employs a monopole type made of steel using a cylindrical shell, and has a structure that a base plate provided at the lower end of the tower shell is fixed to a foundation made of reinforced concrete by an anchor bolt.
FIGS. 11 and 12 show a conventional tower supporting structure. At the lower end of a tower 2, a flange-shaped base plate 22 projected in the horizontal direction from both inner and outer faces of a tower shell 21 is provided. The base plate 22 is a member for coupling and fixing the tower 2 to a foundation B via a number of anchor bolts 10 whose lower ends are embedded in the foundation B. In the configuration example shown in the diagram, two lines of anchor bolts 10 are disposed on a concentric circle on the outer peripheral side and the inner peripheral side of the cylindrical-shaped tower shell 21. By screwing a nut 11 on the upper end side of each of the anchor bolts 10 penetrating bolt holes 22a in the base plate 22, fixing is performed. Reference numeral 12 denotes a layer of grout formed on the concrete of the foundation B.
In a joint structure of a steel column and a steel joist, a method of easily obtaining a clearance with a fixing bracket by slightly moving the joist without requiring subtle crane operation, auxiliary heavy machine, and the like for adjustment is proposed. In the joint structure, both of the fixing bracket for the steel column and the steel joist are cut in a form that they are open to a plumbing side, and a splice plate is attached and fastened with a high-strength bolt, thereby constructing the joint (refer to, for example, patent citation 1).    Patent Citation 1: Japanese Unexamined Patent Application, Publication No. Hei 7-207833
The conventional tower supporting structure has a higher effect by disposing the anchor bolt 10 on the outside of the tower shell 21, so that the base flange often extends toward the outside of the tower shell 21. Further, since the shell member made of steel and having the cylindrical shape is a main strength member, the larger the outside diameter of the tower shell 21 is, the higher a section efficiency is. The monopole tower made of steel is therefore demanded to increase the outside diameter of the shell as much as possible within the range in which the weight can be reduced.
On the other hand, external dimensions of the tower 2 have to satisfy constraint conditions at the time of transportation and the like. In the case of the monopole, the tower 2 is divided into a plurality of sections in the height direction, and the sections are transported. A tower section of a base portion is transported in a state where the base plate 22 is welded and integrated with the tower shell 21. Consequently, the outside diameter of the base plate 22 extended toward the outside of the tower shell 21 becomes the maximum outside diameter of the tower 2. Consequently, it is difficult to increase the outside diameter of the tower shell 21 to the constraint condition.